Lessons learnt from IRSN modelling performed for the 5th EC PCRD BENIPA Exercise

In the framework of the European project BENIPA, the roles and performance of bentonite barriers in geological disposal facilities of radioactive waste has been assessed with regard to safety issues. A specific objective was also to investigate, through numerical calculations, the consistency of performance assessment methods used so far. The present paper summarises IRSN contribution to these tasks of the project in terms of modelling. Process and integrated level investigations have been performed on the basis of a repository design in stiff clay host-rock. Thermo-mechanical simulations have mainly addressed the thermal effect on the expansion of the plastic area around the disposal tunnels, pointing out the role of bentonite regarding temperature profiles and dilation effects. Reactive transport calculations have enabled us to identify the main interaction processes occurring in bentonite/host-rock and bentonite/cement systems, and to assess the consequences on radionuclide migration for various configurations. Integrated level flow and transport calculations have been carried out for a 3D-schematic disposal. Different evolution scenarios were considered leading to diffusive or advective transport, so as to evaluate the efficiency of bentonite buffer and seals, in terms of velocities and release rates in the facility. In addition to an overview of the modelling results, this paper concludes on the bentonite roles, the modelling capacities and the need for improvement with respect to performance assessment. (authors)